The present invention generally relates to self-expanding endoprosthetic devices. In particular, the present invention relates to self-expanding, intraluminal, vascular grafts, generally called stents, adapted to be implanted in a body lumen, such as carotid arteries, coronary arteries, peripheral arteries, veins, or other vessels to maintain the patency of the lumen.
These devices are frequently used in the treatment of atherosclerotic stenosis in blood vessels especially after percutaneous transluminal angioplasty (PTA) or percutaneous transluminal coronary angioplasty (PTCA) procedures, with the intent to reduce the likelihood of restenosis of a vessel. Stents are also used to support a body lumen, tack-up a flap or dissection in a vessel, or in general where the lumen is weak to add support.
For example, during PTCA procedures, it is common to use a dilation catheter to expand a diseased and partially occluded coronary artery so that blood freely flows. Despite the beneficial aspects of PTCA procedures and its widespread and accepted use, it has several drawbacks, including the possible development of restenosis and perhaps acute thrombosis and sub-acute closure. This recurrent stenosis has been estimated to occur in seventeen to fifty percent of patients despite the initial PTCA procedure being successful. Restenosis is a complex and not fully understood biological response to injury of a vessel which results in chronic hyperplasia of the neointima. This neonintimal hyperplasia is activated by growth factors which are released in response to injury. Acute thrombosis is also a result of vascular injury and requires systemic antithrombotic drugs and possibly thrombolytics as well. This therapy can increase bleeding complications at the catheter insertion site and may result in a longer hospital stay. Sub-acute closure is a result of thrombosis, elastic recoil, and/or vessel dissection.
Several procedures have been developed to combat restenosis and sub-acute or abrupt closure, one of which is the delivery and implanting of an intravascular stent. Stents are widely used throughout the United States and in Europe and other countries. Generally speaking, the stents can take numerous forms, however, most common is a generally cylindrical hollow tube that holds open the vascular wall at the area that has been dilated by a dilation catheter. One highly regarded stent used and sold in the United States is sold under the tradename ACS Multi-Link Stent, which is made by Advanced Cardiovascular Systems, Inc., Santa Clara, Calif.
For expandable stents that are delivered with expandable catheters, such as balloon catheters, the stents are positioned over the balloon portion of the catheter and are expanded from a reduced delivery diameter to an enlarged deployment diameter greater than or equal to the inner diameter of the arterial wall by inflating the balloon. Stents of this type are expanded to an enlarged diameter through deformation of the stent, which then engages the vessel wall. Eventual endothelial growth of the vessel wall covers over the stent.
Other stents are self-expanding where the expansion occurs through the properties of the material constituting the stent. Examples of intravascular stents can be found in U.S. Pat. No. 5,292,331 (Boneau); U.S. Pat. No. 4,580,568 (Gianturco); U.S. Pat. No. 4,856,516 (Hillstead); U.S. Pat. No. 5,092,877 (Pinchuk); and U.S. Pat. No. 5,514,154 (Lau et al.).
One problem with some prior art stents, especially those of the expandable type, is that they are often stiff and inflexible. Often, the expandable type stents are formed from stainless steel alloys and are constructed so that they are expanded beyond their elastic limit. Such stents are permanently deformed beyond their elastic limits in order to hold open a body lumen and to maintain the patency of the body lumen. By the same token, since the material is stressed beyond its elastic limit into the plastic region, the material becomes stiff and inflexible.
There are several commercially available stents that are widely used and generally implanted in the coronary arteries after a PTCA procedure. Another class of stents is implanted in vessels that are closer to the surface of the body, such as in the carotid arteries in the neck or in peripheral arteries and veins in the leg. Because these stents are so close to the surface of the body, they are particularly vulnerable to impact forces that can partially or completely collapse the stent and thereby block fluid flow in the vessel. Since these prior art stents are plastically deformed, once collapsed or crushed, they remain collapsed, permanently blocking the vessel. Thus, the prior art stents can pose an undesirable condition to the patient.
Other forces can impact the prior art stents and cause similar partial or total vessel occlusion. Under certain conditions, muscle contractions might cause the prior art stents to partially or totally collapse and to restrict blood flow in the vessel in which they are implanted.
Such important applications as mentioned above have prompted stent designers to use superelastic or shape memory alloys in their stent to exploit the materials"" properties. An example of such shape memory alloy stents is disclosed in, for example, European Patent Application Publication No. EP0873734A2, entitled xe2x80x9cShape Memory Alloy Stent.xe2x80x9d This publication suggests a stent for use in a lumen in a human or animal body having a generally tubular body formed from a shape memory alloy which has been treated so that it exhibits enhanced elastic properties.
The evolution of superelastic and shape memory alloy stents progressed to use of ternary elements in combination with nickel-titanium alloys to obtain specific material properties. Use of a ternary element in a superelastic stent is shown in, for example, U.S. Pat. No. 5,907,893 to Zadno-Azizi et al. As a general proposition, there have been attempts at adding a ternary element to nickel-titanium alloys as disclosed in, for instance, U.S. Pat. No. 5,885,381 to Mitose et al.
Another goal has been to design stents that are capable of easy passage through tortuous anatomies such as those found in a coronary artery. One design entails a nitinol stent having a multiplicity of undulating longitudinal struts that can readily change their lengths in the longitudinal direction so as to provide increased longitudinal flexibility for the stent. An example of such a construction is shown in U.S. Pat. No. 5,879,370 to Fischell et al.
Designing stents for extremely curved and highly tortuous anatomies requires a stent that can bend sufficiently without the struts kinking. To address this kinking problem, one concept is to construct a tubular stent with helically-arranged undulating members having a plurality of helical turns. Linking members formed by rings are laced or interwoven between the undulations in adjacent turns of the helical undulating members. U.S. Pat. No. 6,042,605 to Martin et al. discloses such a construction. The linked undulating elements facilitate bending of the stent.
The foregoing stent designs address the problems with delivering a straight length stent into a tortuous anatomy. These designs, do not, however, address the problems with deploying a straight length stent in an extremely curved vessel. Indeed, when a straight length stent is deployed in a curved vessel, the stent tends to straighten the curved vessel to follow the form of the stent. It is believed that the straightening forces of the stent is damaging to the health of the vessel, may creating emboli, and may generate intimal flaps that promote restenosis.
One possible solution suggests assembling a composite stent piecemeal at the curved vessel delivery site by using short modular sections. This approach is disclosed in U.S. Pat. No. 5,824,037 to Fogarty et al. In this design, modular sections of the prosthesis may be selectively combined to form a composite prosthesis having characteristics that are tailored to the specific requirements of the patient. Each prosthetic module includes one or more standard interface ends for engaging another module, the module interface typically having ends that overlap and/or lock within a predetermined axial range. Selection of the appropriate prosthetic modules and the flexibility of the interface overlap range provide a custom fit intraluminal prosthesis tailored to the individual patient""s needs. The module sections may include bends, although the modules are individually introduced into a lumen system of a patient body so that the composite prosthesis is assembled in situ. Generally, the prosthetic body modules have a variety of selectable body links, bends, and taper characteristics.
Although the foregoing conventional stent designs begin to address the problems with deploying a straight stent in an extremely tortuous or curved anatomy, there is however still a need for a superelastic stent that is specifically intended for use in tortuous anatomies. The present invention satisfies this need.
The present invention is directed to a stent for use in a curved body lumen, comprising a cylindrically-shaped stent including a superelastic alloy, wherein the stent has a unitary construction, and has a length that is greater than a diameter. The superelastic alloy has a low temperature phase that induces a first shape to the stent, and a high temperature phase that induces a second shape with a bend along the length of the stent, and wherein the bend substantially conforms to the curved body lumen.
In a preferred embodiment, the high temperature phase corresponds to an austenitic phase and the low temperature phase corresponds to a martensitic phase. Also, preferably, the superelastic alloy is a nickel titanium composite that may optionally include a ternary element selected from the group of elements consisting of palladium, platinum, chromium, iron, cobalt, vanadium, manganese, boron, copper, aluminum, tungsten, tantalum, or zirconium.
In a preferred embodiment, a nickel titanium or nitinol self-expanding stent can be heat set with various degrees of arch or curvature along its length to accommodate a curved or tortuous vessel anatomy. Therefore, when the stent is deployed in a patient""s body at above the superelastic alloys phase transformation temperature, the stent reverts to its austenitic phase. In this state, the present invention stent assumes its shape with a bend, wherein the bend was heat set to match the curvature of the curved vessel.
Prior art self-expanding nitinol stents that have a straight length, when deployed, exert a continuous radial force on the vessel wall at the deployment site. These prior art stents have a tendency to straighten the lumen regardless of the lumen""s natural curvature. In contrast, the present invention with a heat set curve along its length does not have the same tendency to straighten when inside the curved vessel. Accordingly, trauma to the vessel is minimized and damage to the intima is diminished. Furthermore, the longitudinal bend or bends that are heat set into the present invention stent can vary in both angle and radius of curvature. In various alternative embodiments, the present invention when in the high temperature state may include a curved length that bends in two dimensions, or may have a bend of greater than 90 degrees, or may have compound curves, or any combination of the foregoing.
In the preferred embodiment, the present invention stent is unitary, being fashioned from a single piece of material. The present invention stent is also preferably of sufficient length to have an aspect ratio in which the length is greater than its diameter. This ensures that the stent does not tip within the lumen, and minimizes the chance that the stent may migrate and causing an embolism.
The present invention may optionally include radiopaque markers that assist the physician in proper orientation of the curved stent at the deployment site. In particular, the radiopaque marker may include directional indicia that can be seen in a fluoroscope or by X-ray that help the physician recognize the orientation of the stent. Moreover, the present invention may be delivered by any delivery system and method presently known in the art.
Other features and advantages of the present invention will become more apparent from the following detailed description of the invention when taken in conjunction with the accompanying exemplary drawings.